February 24, 2013

Sunday Sand: dunes, uniformitarianism, and recycling in Utah

There’s only one major sand dune field on the Colorado Plateau, and that can
be found at the Coral Pink Sand Dunes State Park, way down in the southwest
corner of Utah, not far from Zion National Park. And are these dunes really
coral pink? Or reddish orange? Or orangey pink? Opinions vary – as do the
colours of the sand depending on the light and the time of day. But their true
colour hardly matters – they are impressive dunes.

Formed perhaps 10,000 years ago, as the northern ice
was beginning to recede, their sands are home to a diversity of plants and
animals, including the charming – but severely threatened – Coral Pink Sand
Dunes tiger beetle, Cicindela albissima, only recently promoted to a
species in its own right, and which is only found in these dunes.

The sand grains are typical of a windblown heritage, smoothed, frosted, and
rounded by the endless mutual abrasion as they are buffeted by the wind. And
they are all more or less of the same size – the finer grains are blown onwards,
the coarser ones left behind; and they are essentially all quartz.

It’s the local winds that explain why these dunes are where they are. Just to
the north-east of the dunes is a significant “notch” in the ranges of the
plateau. The prevailing winds blast sand south-westwards, are funnelled through
the notch, and, as they emerge and diminish, can no longer carry the sand,
dropping it to feed the dunes. If you look at this on Google Earth, you can
almost feel the sands streaming through the gap in the hills:

And why is that gap there? Well, because one of the great geo-architectural
features of the Colorado Plateau, the Sevier Fault, cuts through it. The fault
is a huge fracture in the earth’s crust, over a hundred kilometres long, and
almost vertical; everything to the west of the fault has dropped down 700 meters
(2300 feet) relative to the rocks on the east. The fault is clearly visible on
the satellite image below, right, as a scar slicing across the landscape:

On the left is exactly the same area but shown as a geological map, the rocks
of different ages appearing in different colours, the faults as black lines. To
the east of the Sevier fault, the landscape is dominated by the light blue rocks
of the Navajo Sandstone, to the west, the green of the Carmel Formation, mostly
made up of limestones. The Navajo was deposited during the Lower Jurassic,
around 185 million years ago, the Carmel later, on top of it, around 165 million
years ago. So the map shows clearly that the movement on the Sevier fault is
down to the west – the rocks of the Carmel are younger than the Navajo in the
hills to the east, and have been completely removed by erosion on the uplifted
side of the fault.

It’s a big and complex fault system, and the grinding of fault movement would
have pulverised the rock along the fault zone, leaving it weaker and exposed to
erosion – hence the gap through which the sands blow. And the Sevier fault is
still on the move – it has been seismically active for a long time, part of the
so-called intermontane
seismic belt.

So, we have figured out why all those sand grains are piled up where they
are, but what’s their story? The photo below shows the dunes in the foreground
and, in background, the pink sandstones of the Navajo Sandstone on the east side
of the fault. Look carefully and you will recognise dunes ancient and
modern.

The Navajo is an iconic formation, the star of the landscapes of many
of Utah’s national and state parks. It represents an enormous erg, a large sand
sea that extended over most of Utah as well as parts of New Mexico, Arizona,
Colorado and Wyoming. Look carefully at the bluffs in the photo, and you can see
gigantic scale cross-bedding, signalling wind-blown sand, dunes. Excavate
the dunes of the Coral Pink Sand Dunes (in your imagination), and exactly the
same structures will be revealed – the vital principle of uniformitarianism at
work. That principle derives from the observations of James Hutton in the late
18th century, that the processes we see today sculpting the surface of our
planet have always operated, and always will. Commonly summarised as “the
present is the key to the past,” it has been debated and refined ever since –
for example, the rates at which a process, e.g., global
volcanism, operated have varied over time – but the principle is, indeed, the
key to our being able to read the ledgers of earth’s history. Winds have always
blown, sand has always been on the move, dunes have always been built in the
same way.

The bluffs of ancient Navajo dunes in south-western Utah have been exposed to
the attack of weathering and erosion for a long time, disintegrating and
liberating the tough little sand grains. Those grains have been swept along by
rivers, left in sand banks, dried out, and picked up by the wind for the next
stage of their travels – to the Coral Pink Sand Dunes State Park. There they
rest, for a while, on their never-ending journey, and what you see in those
dunes today are re-cycled grains from a desert nearly two hundred million years
ago.

[Heartfelt thanks to Myrna and Charles Gifford for collecting and sending the sand,
and for the photographs.